There were 27 people riding on a train before it stopped and some number of people got off. Now there are 7 people riding on the train. How many people got off the train at the stop?
Solution: The number of people who got off is the difference between the number who were riding before and the number who are riding now. The difference is $27 - 7$ people. $27 - 7 = 20$ people got off.